Nearly all manufactured objects need to be inspected after they are fabricated. A variety of optical devices have been developed for in-fab and post-fab inspection. Many of these optical devices scan the surface of the part and are able to determine the surface profile of the part with good accuracy. However, as the accuracy and tolerance requirements of the part become tighter, the measurement accuracy, precision, and repeatability of the scanning optical device must be improved accordingly. As a rule of thumb, the measurement device should be at least ten times better than the required surface figure so the errors of the measurement device have a negligible impact on the overall error budget.
One way to reduce the measurement errors of the scanning optical device is to build the scanner from components that themselves have extremely tight tolerances. Unfortunately this approach will drive up the cost of the scanner and make it uneconomical for use in an in-fab or post-fab inspection environment.
A second way to reduce the measurement errors of the scanning optical device is to build the scanner from components that have nominal tolerances, and then measure or otherwise calibrate the components of the system and merge the calibration results into an overall calibration algorithm. Typical components to be calibrated include the scanning drive electronics and mechanism (offsets, gain, and nonlinearities in both scan axes), the imaging lens (magnification, distortion, and non-telecentricities), and the effects of the placement errors of components in the illumination arm of the scanner. Characterizing and calibrating all of these quantities individually and then subsequently mathematically combining them into a single calibration formula is difficult and time-consuming. Furthermore, if a quantity is inadvertently omitted from the process, then the calibration will be incomplete and the accuracy of the scanner will be compromised.
Yet another way to minimize the measurement errors associated with the scanner is to provide a closed-loop feedback mechanism that can be used to measure the actual scan location, and provide real-time corrections to the scanner to ensure that the actual scan location is the same as the desired scan location. However, the feedback mechanism generally entails additional cost due to the inclusion of the feedback components (e.g., mirrors, electronics, lenses, image sensors), and equally important will increase the size or volume of the optical scanner. If the scanner must be compact so that it can fit into or measure small recesses of a part, then the feedback approach may not be viable.